A plane chartered by the British government is to deport about 100 Tamil asylum seekers to Sri Lanka, according to a human rights charity.
Human Rights Watch claims the aircraft will fly the Tamils back to the country where they could face interrogation and torture.
It claims at least eight similar flights have flown Tamils back to Colombo in recent months, and several of those deported have gone on to face serious abuse by Sri Lankan army forces.
The charity says that one Tamil alleged that during interrogation he was beaten with batons and burned with cigarettes, and had kerosene poured over his head.
It details allegations by other Tamils, some of whom claim to have been subjected to gang rape by security forces.
"The British government has an international legal obligation not to deport people who have a credible fear of torture upon return," said Brad Adams of Human Rights Watch.
"Convincing reports of arbitrary arrests and torture demand that the UK government suspend returns of rejected Tamil asylum seekers to Sri Lanka until it can fairly and thoroughly assess their individual claims based on up-to-date human rights information on Sri Lanka."
Deportation flights have continued despite an MP last year accusing the government of "painting targets on the backs" of Sri Lankan returnees.
Siobhain McDonagh, MP for Mitcham and Morden, told the Commons the British Government is complicit in the torture of Tamil civilians as it continues to deport many people seeking asylum in the UK.
"The British Government is supposed to be one of the leading forces in the Commonwealth. Yet it is not only turning a blind eye, it is sending planeload after planeload of Tamils back," she said last June.
And last September, a plane chartered by the British Government to return failed asylum seekers to Sri Lanka landed in Colombo following the failure of last-minute legal efforts to prevent it leaving.
At that time, shadow foreign secretary Douglas Alexander said he had written to foreign secretary William Hague asking for assurances that 50 Tamils returned to Sri Lanka would not be tortured.
The latest deportation came as hundreds of demonstrators, led by ruling party local government councillors, marched to the United States embassy in protest against the US decision to support a UN human rights resolution.
The US earlier announced its decision to back a resolution at the Human Rights Council in Geneva asking the Rajapaksa government to submit a time table to implement recommendations made by a government appointed commission to improve good governance, human rights and reconciliation with minority Tamils.
The Indian Ocean island nation, which ended its quarter century Tamil separatist war in May 2009, is under heavy pressure from western nations and international rights groups to investigate alleged war crimes in the final phase of the war.